With the process list model being publicly available from the IDevice
interface, there's no longer a need to force all subclasses to create
the dialog themselves via opaque handles.
Since clients can now check whether a device offers a process list,
the base process list model has been made abstract.

Change-Id: If4a0aa68a95b221862c287ad8397ebabe9be5909
Reviewed-by: hjk <qthjk@ovi.com>